Kipp&Zonen BSRN Scientific Solar Monitoring System - Manual (Page 39)

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Figure 3.3. The support structure used to elevate instrum ents above the
local horizon. The structural steel and concrete support structure at the
Bratt’s Lake Observatory, Meteorological Service of Canada.
provided. This can vary from a perm anent deck structure to a sim ple step ladder, rem em bering
that the easier the access to the instrum ent the m ore likely the instrum ent will be well m aintained.
If the instrum ent is to be m ounted on the roof of a building care m ust be taken to guarantee that
the instrum ent will not be blown off during high winds. The secure anchoring of the instrum ent
stand should be done in consultation with the building m anager or engineer. If possible, a
perm anent installation with the instrum ent stand bolted to the building is preferable to the use of
stands set on the roof and secured only by heavy weights.
Depending on the site, further m easures m ay be required to ensure the stability of the
pyranom eter platform during high wind conditions. Extra guy-wires or bracketing m ay be added to
keep the stand from oscillating.
3.5.3 Cables
3.5.3.1 Signal cables
Just as im portant as determ ining the best field of view for the instrum ents, is the routing of the signal
cable from the instrum ent to the data acquisition system . As m ost surface-based radiom eters are therm opile
instrum ents, the m axim um signal is usually in-the-order of 10 m V for a 1000 W m flux or 10 :V W
-2 -1
m . Such sm all signals can be affected easily by large line resistance, due to long cable lengths, and 2
electrical interference from other sources, particularly AC power lines running parallel to the signal
lines. Several suggestions follow to aid in the design of the m easurem ent system .
(1) All signal cables should be twisted wire configured as a ground and signal pair sheathed in
a foil wrap. The outer sheathing of the signal cable should be based upon the clim atic regim e
of the station and the overall EMF to which the cable is to be subjected. It is recom m ended
that cables be m ade of stranded copper for flexibility.
(2) Cable lengths should be kept as short as practically possible. The overall length of the cable
is dependent upon the rem oteness of the m easurem ent platform and the type of data acquisition
system being used to sam ple the signal. Types of data acquisition system s are discussed
in Section 6.
W here long cables are required and the total resistance of the cable is greater than 10 S
(approxim ately 50 m ), a pre-am plifier should be placed at the instrum ent end of the system .
Extrem e care should be taken with this solution because of the tem perature dependency
and non-linearity of electronic com ponents.